Easiest might be just to drag-and-drop tabs from one instance of Firefox onto the other. That gives you control of which tabs to move and in what order they appear.
Of course, if there areA multiple tab selection feature was added to Firefox in version 64. If you have a great manylarge number of tabs opento move and want to do it in both instancesbulk, this might beselect the first tab, then shift-select the last tab to select the entire range (you can also Ctrl-select individual tabs to create a nuisancecollection of selected tabs). Then drag the lot to the other window. So moving all tabs from one window to the other can be done with three mouse clicks.
